Atomic layer deposition (ALD), also known as atomic layer epitaxy, is a process for depositing highly uniform and conformal thin layers of a metal on a surface. The surface is exposed to vapors of the metal precursor and a reducing agent. Such films have a wide variety of applications in semiconductor microelectronics and optical films. The conventional ALD process, which uses a two-step procedure, is described by M. Ritala and M. Leskela in “Atomic Layer Deposition” in Handbook of Thin Film Materials, H. S. Nalwa, Editor, Academic Press, San Diego, 2001, Volume 1, Chapter 2. Variations of the process have been used to deposit metal-containing layers.
In the typical two-step ALD process, there is a self-limiting adsorption of the metal complex to the surface that is controlled by the interaction of the precursor with the substrate itself in a thermal degradation step. The loss of the ligand is induced thermally, as the metal surface has no functional groups to induce such reactions chemically. It is desired that the metal precursor be stable enough to be transferred into the deposition chamber, but reactive enough to undergo a transformation at the substrate surface.
In a related ALD process the substrate contains functional groups that control the process. These functional groups react chemically with at least one ligand on the metal-containing precursor. For example, the standard process used to prepare conformal Al2O3 films uses a substrate with hydroxyl groups. The substrate is contacted with Al(CH3)3, which reacts with the surface hydroxyl groups to form an adsorbed Al—O complex with the liberation of methane. When the surface hydroxyl groups are consumed, the reaction stops. Water is then contacted with the Al—O complex on the surface to generate an aluminum oxide phase and additional hydroxyl groups. The process is then repeated as needed to grow an oxide film of desired thickness. The deposition rate of the Al(CH3)3 is controlled by the number of surface hydroxyl groups. Once the hydroxyl groups are consumed, no additional Al(CH3)3 can be adsorbed to the surface.
In the deposition of metal films, there is no reactive group on the substrate surface to initiate the type of self-limiting reaction that occurs in the Al2O3 case. In these instances, the thermal degradation method is used. For example, in the deposition of a tantalum barrier layer on a tantalum nitride substrate, the self-limiting adsorption is achieved through the thermal decomposition of the tantalum precursor. The tantalum precursor is preferably designed to have the volatility and stability needed for transport to the reaction chamber, but also the reactivity to undergo clean thermal decomposition to allow a metal complex to chemisorb to the substrate surface and to result in tantalum films that are not contaminated with fragments from the tantalum ligands degraded during the thermal deposition.
Tantalum-containing films are useful in integrated circuits and, in particular, tantalum and tantalum nitride films have been used as barrier films.
The processes of the present invention provide a relatively low temperature process for the formation of high quality, uniform tantalum-containing films and provide novel tantalum complexes that that can be used as tantalum precursors in deposition processes.